It is heretofore been known in the art to provide head-up display systems in vehicles for projecting information related to the operation of the vehicle over the real world view through a windshield. The reason for this display is to enable the operator of the vehicle to view the information supplied to the head-up display without removing his eyes from the real world as viewed through this windshield.
Heretofore the combiner used with the heads up display has been separate from the windshield. It would be advantageous to include the combiner for the heads up display in the windshield, thereby minimizing the amount of supporting structure required, and reducing the potential for obstruction of the view of the outside world. It would further be advantageous for the combiner to be constructed in such a way that the edges are not visible to the user.
It is heretofore known in the art to use holographic optical elements for the combiners and such head up displays. A holographic optical element is a generally layered device having an interference pattern therein. In accordance with the known art holographic optical elements are ordinarily constructed of dichromated gelatin which has been exposed to an interference pattern of coherent light and developed. Such a dichromated gelatin layer when developed embodies the interference pattern in varying indices of refraction. These holographic optical elements can be constructed so as to efficiently reflect light of particular wavelengths while not obstructing the visibility through the device at other wavelengths. Thus, for example, the display of a cathode ray tube phosphor having a particular wavelength can be made to be reflected to the user via such a holographic optical element, while the holographic optical element provides minimum obstruction of other wavelengths passing through.
There has heretofore been no convenient manner of forming such a holographic optical element into the windshield of the vehicle, nor of reducing the visibility of the edges of such a holographic optical element to the user during ordinary use.